New error in .NET 8 Android projects

[jonathanpeppers]

Starting in this Maestro bump: dotnet/android#7630

We started seeing:

ILLink error IL1034: Root assembly 'MyAndroidApp, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' does not have entry point.

Might be caused by: #3124 (comment)

Android apps are class libraries, because there is no static void Main() method.

To work around this I did:

  <Target Name="_FixRootAssembly" AfterTargets="PrepareForILLink">
    <ItemGroup>
      <TrimmerRootAssembly Update="@(TrimmerRootAssembly)" Condition=" '%(RootMode)' == 'EntryPoint' " RootMode="Library" />
    </ItemGroup>
  </Target>

Filing an issue here, to see if this is what we should actually do -- or something else.

[vitek-karas]

@sbomer please take a look.

[sbomer]

I think the workaround is doing the right thing (edit: see my next comment), and I think it would make sense to put the fix for this into the SDK. In #3131 we decided against it, but I think this issue shows that we really do already have non-exe projects as a supported scenario - I just didn't know it.

I think we should fix this by using RootMode="library" RootMode="visible" when the OutputType is Library. From dotnet/android#4604, I think this will work for the Android scenario. Does that sound right to you @jonathanpeppers?

Note @vitek-karas @marek-safar @MichalStrehovsky this will make "library" "visible" root mode an officially supported feature for the first time - are we OK with this? If not, the alternative would be to use "all" to get the behavior from before #3124.

[sbomer]

Sorry, I just remembered that "library" has extra behavior which disables optimizations for when we are only trimming a single library against reference assemblies. I think what we actually want is "visible". @jonathanpeppers you might want to use RootMode="Visible" (or "All", to get the previous behavior) in your workaround instead, until we resolve this issue.

[sbomer]

Another option, instead of using the assembly RootMode, would be to let the Android SDK tell the linker via XML descriptor which entry points to keep. Presumably there is an effective entry point (or multiple) for Android apps that is statically known, even if it's not encoded as such in the PE file. This option has slightly cleaner trimming behavior, but I don't like that it relies on the XML.

[MichalStrehovsky]

Note @vitek-karas @marek-safar @MichalStrehovsky this will make "library" "visible" root mode an officially supported feature for the first time - are we OK with this? If not, the alternative would be to use "all" to get the behavior from before #3124.

It's not clear to me what "publishing a self-contained library" actually means. (We only support trimming for selfcontained deployments.)

E.g. if we want to support dotnet/runtime#79377 with IL Linker, we would actually only want to root things with [UnmanagedCallersOnly(EntryPoint = "Foo")]. This is a definition for "publishing a self-contained library" that makes sense.

@jonathanpeppers what are the trimming roots in an Android application?

[marek-safar]

Note @vitek-karas @marek-safar @MichalStrehovsky this will make "library" "visible" root mode an officially supported feature for the first time - are we OK with this? If not, the alternative would be to use "all" to get the behavior from before #3124.

I'm not sure that's what we really want. My understanding of the scenario is that this is not library mode trimming but trimming with a custom entry point name that is hardcoded somewhere in XA interop. I think the expectation is that all public methods in such library will be trimmed.

[jonathanpeppers]

Android has many roots, where Android/Java will call into managed code, such as:

• Android.App.Application
• Android.App.Activity's
• Services
• Broadcast receivers
• Content providers
• Views created from Android .xml
• Probably more, I'm not thinking of

I think Android tried really hard to be "decoupled", where these types don't necessarily have references to each other.

The solution we brought over from Xamarin.Android, is anything that subclasses Java.Lang.Object is preserved by a custom linker step. So, I would expect most Android apps have most of the main assembly preserved. It sounds like [UnmanagedCallersOnly] would do something similar.

[jonathanpeppers]

/cc @jonpryor you might find this thread interesting.

[MichalStrehovsky]

If there's a custom step that roots everything necessary, maybe the fix would be not to root anything and leave it to the custom step? Basically any extra rooting the TrimmerRootAssembly can bring would probably be just garbage we could have collected during trimming.